This invention relates to a torque assist method and apparatus to improve image registration in an electrophographic imaging system, but more specifically, To an auxiliary belt drive that overcomes unwanted drags and/or system loads.
Electrophotographic printing machines employ photoreceptor members, typically in the form of a belt that is electrostatically charged to a potential so as to sensitize the surface thereof. The charged portion of the belt is exposed to a light image of an original document being reproduced. Exposure of the charged member selectively dissipates the charge thereon in the irradiated areas to record an electrostatic latent image corresponding to the informational areas contained within an original document. After the electrostatic latent image is recorded on the photoreceptor member, a developer material is brought into contact therewith to develop the latent image. The electrostatic latent image may be developed using a dry developer material comprising carrier granules having toner particles adhering triboelectrically thereto or using a liquid developer material. Toner particles are attracted to the latent image, forming a visible powder image on the surface of the photoreceptor belt. After the electrostatic latent image is developed with the toner particles, the toner powder image is transferred to a substrate, such as a sheet of paper. Thereafter, the toner image is heated to permanently fuse the image to the substrate.
In order to reproduce a color image, the printing machine includes a plurality of imaging stations each of which deposits a toner of a giver color. Each station has a charging device for charging the photoreceptor surface, an exposing device for selectively illuminating the charged portions of the photoreceptor surface to record an electrostatic latent image thereon, and a developer unit for developing the electrostatic latent image with toner particles. Each developer unit deposits different color toner particles on the electrostatic latent image. The images are developed, at least partially, in superimposed registration with one another to form a multi-color toner powder image. The resultant multi-color powder image is subsequently transferred to a substrate. The transferred multi-color image is then permanently fused to the sheet forming the color print. To obtain a high quality image, registration of the images at each of the developer stations is essential.
Registration is achieved by accurately positioning the photoreceptor belt at the various imaging and developing stations along the belt path using a drive mechanism that typically comprises drive rollers that advance a substrate along the path and backer bars that support the belt. Many such drive rollers have a coating commercially known as an EPDM elastomer that is applied to the surface thereof to improve friction coupling between the drive mechanism and the belt. Due to backer bar and subsystem drag, the drive rollers often experience slippage the photoreceptor belt and at other locations along the belt when the surface of the drive roller encounters particle contamination. Slippage has a deleterious impact on image registration, particularly when latent images are applied at multiple imaging stations.
An auxiliary belt drive may address slippage problems, but in order to be effective, the torque level and proper location of the auxiliary drive is essential to attain optimum drive benefit while at the same time satisfying motion quality and registration requirements of the imaging system. In addition, belt tensioning and drive capacity requirements must also be met.
In accordance with one embodiment of the invention, a belt drive module that achieve the above and other goals comprises a belt that moves along a path, at least one support roller that supports the belt along the path, a drive roller that effects movement of the belt along the path, a tension roller that applies a tension force on the belt in order to maintain engagement of the belt with the drive and support rollers, at least one processing station (e.g., an image processing station) disposed along the path to perform a process relative to a position of the belt, and a torque assist drive that applies a torque assist force Td at a location between the drive roller and the tension roller.
In accordance with another aspect of the invention, a method of providing a torque assist force Td to a belt in a belt drive mechanism comprises providing a roller support structure that guides movement of the belt along a predetermined path that includes processing stations, applying a drive force to rotate the belt along the path, applying a tension force to a slack side of the belt relative to the drive force in order to maintain tension during movement of the belt along the path, and providing a torque assist force Td to the belt at a location between the drive force and the tension force.
Advantages provided by the invention include reduced drive roll maintenance. With torque assist, periodic cleaning of the drive roll in the field is reduced. In addition, catastrophic failures may be avoided. For example, should a sudden change in contamination level occur during operation of the belt drive mechanism, the torque assist drive provided herein is robust against a low friction coefficients on the drive and torque assist roll surfaces thereby to prevent a catastrophic failure due to, for example, contamination or other debris.
Other features of the invention include providing a constant torque friction clutch or a current limited DC motor to provide the torque assist force. The invention, though, is pointed out with particularity by the appended claims.